The present invention relates to a method of producing with good productivity polymer particles having superior powder characteristics from polymer latex obtained by emulsion polymerization, and more particularly, to a production method of polymer particles having high bulk specific gravity, few coarse particles and fine particles, and powder characteristics optimum for shock resistance modifiers such as vinyl chloride resin.
The present application is based on a patent application filed in Japan (Japanese Patent Application No. 11-246743), and the described contents of the Japanese application are incorporated as a part of the present specification.
Polymer particles are typically obtained from polymer latex obtained by emulsion polymerization using a method in which acid or salt is added to latex in a stable state to coagulate the polymer and form a slurry containing the coagulated polymer followed by dehydration and drying to obtain polymer particles.
This coagulation step is required to prevent problems such as retention and obstruction of solids in steps following the coagulation step, and result in the formation of polymer particles having superior powder characteristics so as to allow the carrying out of stable industrial production.
The particle size distribution of particles in the coagulation step is particularly important. If there are numerous coarse particles, not only can these particles cause problems such as retention and obstruction in the line and in the drying machinery, but they may also cause defective dispersion during following processing as well as a poor molded appearance as a result being mixed into the finished product On the other hand, an excessively high number of fine particles can cause worsening of the working environment and decreased ease of dehydration due to the generation of dust, consolidation of individual particles during storage, namely blocking phenomenon, or decreased fluidity, resulting in cases in which these fine particles can have an effect on not only the production stability of the process, but also on product quality. Since rubber-like polymer latex is particularly susceptible to the generation of coarse particles, it has been difficult to solve such problems.
In addition, since particles having low bulk specific gravity are coarse within the particles, particle strength is inadequate. Consequently, such particles resulted in the formation of fine particles due being destroyed in steps following the coaguation step, or were caused increases in transportation costs due to their high bulk even after obtaining the finished product. Moreover, there were also cases in which these particles having low bulk specific gravity had an effect on product quality as a result of causing blocking phenomenon due to the formation of fine particles during storage, thereby creating extremely serious problems in terms of industrial application.
In order to solve these problems, methods have been proposed that include a method in which polymer particles containing few coarse particles and fine particles and having a sharp particle size distribution are recovered from polymer latex by gentle coagulation (Japanese Examined Patent Application, Second Publication, No. 3-51728 and Japanese Unexamined Patent Application, First Publication, No. 5-320221), and a method in which spherical polymer particles are obtained by stirring after adding an organic solvent liquid that is insoluble in a dispersion medium and a poor solvent of the polymer (Japanese Unexamined Patent Application, First Publication, No. 62-149726, Japanese Unexamined Patent Application, First Publication, No. 62-115032).
As a result of these technologies, polymer particles having a narrow particle size distribution and superior powder characteristics were able to be obtained even with rubber-like polymers.
However, in the case of the former method, in order to reduce the amount of coagulant used to coagulate the polymer latex, the polymer particles formed in the coagulation tank contain a large amount of water, resulting in the problem of excessive costs in terms of equipment and energy for their dehydration and drying. In addition, in the case of the latter method, since organic solvent ends up penetrating into the rubber portion of the polymer depending on the type of polymer, there was the problem of being unable to obtain polymer particles having superior powder characteristics unless a large amount of organic solvent is used, while also having the possibility of causing environmental contamination. In addition, even if the organic solvent was able to be completely recovered and reused, this would result in problems in terms of costs due to the need for a considerable investment in equipment for that purpose.
The development of a production method that does not place a burden on the environment and is cost competitive is currently becoming extremely important industrially, and the methods of the prior art were unable to yield satisfactory results with respect to these points.
The object of the present invention is to provide a method of obtaining particles having superior process clearance and powder characteristics, namely particles having high bulk specific gravity that contain few coarse particles and fine particles, with an ordinary stirring tank or other comparatively inexpensive equipment and without using organic solvent and the like.
As a result of conducting diligent studies, the inventors of the present invention found that, in order to obtain graft polymer particles having high bulk specific gravity in a coagulation process, it is effective to employ a method comprising discharging polymer latex into a stirring tank at a low linear velocity from an immersed nozzle having a discharge portion with a relatively large cross-sectional surface area, contacting with a coagulant to rapidly coagulate the graft polymer and obtain a slurry liquid, and then solidifying the slurry liquid.
Moreover, it was also found that, in order to obtain particles containing few coarse particles and fine particles that have superior process clearance and powder characteristics, it is important that the slurry liquid obtained in the coagulation step be in the form of a cream having a high solid concentration that is free of coarse particles.
Namely, the production method of polymer particles of the present invention is a method for producing graft polymer, particles comprising contacting a coagulant with a polymer latex (A) obtained by graft polymerizing a monomer containing methylmethacrylate to a rubber-like polymer, the method having a coagulation step in which polymer latex (A) is discharged into a stirring tank from an immersed nozzle provided so that the cross-sectional surface area of the discharge portion is 40 mm2 or more and the direction of discharge is facing in the same direction as the flow in the stirring tank and so that the linear velocity at the nozzle outlet is a velocity of 50-350 mm/s, and contacted with a coagulant to coagulate the graft polymer and obtain a slurry liquid; followed by a solidification step in which the resulting slurry liquid is held at a temperature of 60-100xc2x0 C. to solidify the coagulated graft polymer.
According to the production method of the present invention, graft polymer particles can be obtained containing few fine particles and having high bulk specific gravity and superior powder characteristics without requiring the use of organic solvent or special equipment Accordingly, polymer particles having superior powder characteristics can be produced at low cost and in a stable manner.